Millions of gallons of used motor oil (or lubricating oil) are generated each day in the US, and a vast majority of used oil is improperly disposed each year. Lubricating oil, which is essentially a mixture of paraffinic, naphthenic, and aromatic hydrocarbons, is stable and does not degrade or wear out. However, additives, including both inorganic and organometallic additives, that are added to enhance motor oil, such as to improve its lubricating and anti-wear properties, oxidize, deplete, and wear out. Consequently, used motor oil needs to be treated to remove worn-out additives prior to recycling it as a feedstock for preparing fresh motor oil.
Refining of used motor oil began in earnest in the 1970s. Early processes involved removing contaminants such as dirt, water, fuel, and additives by a wide variety of unit operations including dehydration, distillation, treatment with chemicals and clays, solvent extraction, and solvent de-asphalting. More recently, however, many more additives have been developed and are used in improving properties of the motor oil. The total amount of additives currently being added to prepare fresh motor oil used in the US is estimated to be close to 20 weight percent. Due to the costs associated with recovery of solvent, inability of clays to remove modern additives, cost associated with disposal of spent clays and chemicals, and production of re-refined oil with inconsistent quality, such processes have fallen out of favor. Therefore, a process for refining used motor oil containing high percentages of additives is desirable.